Transport Canada recommends that passengers heading to the United States arrive 3 hours prior to their flight time to allow for security screening. --- gtaa.com, Jan 21, 2010
Flying from Toronto to the United States used to be easy because you'd clear US immigration before boarding. You'd then arrive as if you took a US domestic flight.
That changed on December 26, 2009.
This podcast accompanies a blog post at http://blog.riscario.com/2010/01/why-we-accept-airport-security-but-deny.html